The Same Pitch, Completely Different Sound
Sing the same note twice -- first on 'ah' (as in 'father'), then on 'oo' (as in 'boot'). The pitch is identical, but the sound is dramatically different. That difference is timbre, and it is controlled primarily by the shape of your vocal tract. Your tongue position, jaw opening, and lip rounding all change the resonant properties of your mouth and throat, filtering the sound produced by your vocal folds in different ways.
The 'Ah' Vowel: Open and Bright
An 'ah' vowel is generally produced with a more open mouth shape than an 'oo' vowel, although exact tongue and jaw positions vary by language, singer, and pitch. Its formant pattern gives it a recognizably open quality. Blob Opera lets horizontal or depth-oriented movement change vowel color; the safest way to learn the direction in the current version is to move slowly and listen.
The 'Oo' Vowel: Rounded and Mellow
An 'oo' vowel normally involves lip rounding and a high back tongue position, producing a lower formant pattern than an open 'ah'. In Blob Opera, changing the non-pitch direction changes the vowel sound while the vertical position controls pitch. The visual direction can be described as forward and backward in Google's guide, even though a flat screen may make it feel horizontal.
Practical Exercises
Select just one blob and slowly drag it from far left to far right while keeping the pitch constant. Listen to how the timbre changes. On the left, you hear a bright, open 'ah.' On the right, a soft, rounded 'oo.' Now do the same with a different blob to hear how the same vowel transition sounds at a different pitch. This is one of the best ways to train your ear to hear vowel shapes, because you isolate the variable being changed while keeping everything else constant.